A radar apparatus is known generally as an apparatus for measuring positional information or the like of a remotely located object. The radar apparatus emits waves such as electromagnetic waves or acoustic waves into the space, receives the waves reflected from a target, and analyzes the received signals. In this way, the radar apparatus measures the distance from the radar apparatus to the target or the angle of the target relative to the radar apparatus. One such radar apparatus is a meteorological radar apparatus whose targets are solid particles (aerosols) floating in the atmosphere. This meteorological radar apparatus measures the moving velocity of the aerosols, namely “wind velocity,” from the amount of phase rotation or frequency shift of the waves reflected from the particles.
Among the meteorological radar apparatuses, a laser radar apparatus (lidar apparatus) which uses light as electromagnetic waves to be emitted is known. The lidar apparatus emits an extremely narrow beam and can observe a target with a high angular resolution. The lidar apparatus is therefore also used as an anemometer for measuring the wind direction and the wind speed. In order to measure a wind velocity vector representing a wind direction and a wind speed, generally the VAD (Velocity Azimuth Display) method or vector operation method is used as a method for calculating the wind velocity vector. According to the VAD method, a laser light is emitted in multiple directions, and the line-of-sight velocity representing the value of the wind speed in a line-of-sight direction, which is the direction in which a laser light is emitted, is measured.
In the case where the lidar apparatus is used for measuring a wind velocity vector, the lidar apparatus is installed generally outdoors. When it is intended to measure a horizontal distribution of wind velocity vectors each representing a wind velocity at a distance from the lidar apparatus, it is necessary to emit the laser light horizontally from the lidar apparatus. When it is intended to measure a vertical distribution of wind velocity vectors, it is necessary to emit the laser light vertically. A particular problem with the measurement of the vertical distribution of wind velocity vectors at different altitudes is caused by “precipitation.” During precipitation, aerosols are present and therefore the wind velocity vectors can be measured. The laser light is emitted in the vertical direction and accordingly the lens surface of a scanner emitting the laser light is also oriented vertically. Due to this, water droplets stay on the lens surface. A resultant phenomenon is that the intensity of the emitted laser light is attenuated by the water droplets and the SN ratio (Signal-to-Noise ratio) is lowered. The observable altitude is therefore lowered. In other words, the data acquisition ratio is lowered.
When lowering of the SN ratio is detected at least a part of the lens surface, a conventional lidar apparatus for measuring a vertical distribution of wind is configured to spray a washer fluid from a washer fluid tank of the lidar apparatus onto the lens surface, and to drive a wiper to wipe the lens surface (see for example Patent Document 1).